Double fired u-tube fired heater

ABSTRACT

A fired heater is provided with u-shaped coils is provided. The coils have at least one inlet, at least one inlet section, at least one outlet and at least one outlet section. Burners are provided in both the center of the end wall of the fired heater and on the floor of the side wall. The fired heater is more compact and provides more even heat distribution compared to prior art heaters.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Provisional Application No.62/161,792 filed May 14, 2015, the contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a fired heater for providing heat for a reactorfor heating a hydrocarbon feed. More particularly, the invention relatesto improved fired heaters for use in catalytic reforming units toproduce desired products from feeds such as naphtha or in producinglight olefins.

Catalytic reforming reactors that mostly consist of hydrocarbons withmore than six carbon atoms are processed in a catalytic reformer anddepending upon the conditions used can be converted to aromaticcompounds or cracked to make paraffins. Fired heaters are used asnecessary to heat the feed to a reaction temperature such as about 500°C.

Processes for converting hydrocarbons at higher temperature have beenknown for many decades. U.S. Pat. No. 2,182,586, describes a reactor andprocess for the pyrolytic conversion of a fluid hydrocarbon oil. Use ismade of a horizontally arranged single reactor pipe (the publicationrefers to “tubes”, but these are connected in a serial flow connectionand thus form in fact a single tube), which results in relatively longresidence times which are common in the process of thermal cracking ofliquid hydrocarbon oils to improve motor fuel quality such asvisbreaking. The use of the described heater for a process like steamcracking or for the cracking of a vaporous feed is not mentioned.Rather, excessive cracking and excessive gas formation are avoided.

U.S. Pat. No. 2,324,553, published in 1943, shows another heater for thepyrolytical conversion of hydrocarbons, wherein the reactor pipe isformed of serially connected “tubes”, which are horizontally positionedin the heater. In the described process, oil is passed through the tubeto a temperature below an active cracking temperature.

WO 97/28232 describes a cracking furnace for thermally cracking a liquidhydrocarbon feed in a spiral pipe. The furnace is said to have a reducedsensitivity for coke formation and an increased liquid residence time.It is not disclosed to use the installation for steam cracking.

Steam cracking is a specific form of thermal cracking of hydrocarbons inthe presence of steam with specific process kinetics and other processcharacteristics. Herein, the hydrocarbon feed is thermally cracked inthe vapor phase in the presence of steam. The cracking is carried out atmuch higher severity than applied in the moderate cracking of liquidhydrocarbon oils to improve fluid quality. Steam cracking furnacescomprise at least one firebox (also known as a radiant section) whichcomprises a number of burners for heating the interior. A number ofreactor tubes (known as cracking tubes or cracking coils) through whichthe feed can pass are disposed through the firebox. The vapor feed inthe tubes is heated to such a high temperature that rapid decompositionof molecules occurs, which yields desired light olefins such as ethyleneand propylene. The mixture of hydrocarbon feed and steam typicallyenters the reactor tubes as a vapor at about 600° C. In the tubes, themixture is usually heated to about 850° C. by the heat released byfiring fuel in the burners. The hydrocarbons react in the heated tubesand are converted into a gaseous product, rich in primary olefins suchas ethylene and propylene.

In cracking furnaces, the reactor tubes may be arranged vertically inone or more passes. In the art, the term cracking coil is also used. Oneor more of the cracking coils, which may be identical or not identical,may be present to form the total radiant reactor section of a firebox.Conventionally, ethylene cracking tubes are arranged in the firebox inone lane wherein the lane is heated from both sides by burners.

Such a lane may be in a so-called in-line arrangement whereby all thereactor tubes are arranged in essentially the same vertical plane.Alternatively, the tubes in such a lane may be in a so-called staggeredarrangement whereby the tubes are arranged in two essential verticalparallel planes whereby the tubes are arranged in a triangular pitchtowards each other. Such a triangular can be with equal sides (i.e.equilateral triangular pitch) or with unequal sides which is called anextended pitch.

SUMMARY OF THE INVENTION

The invention provides a fired heater comprising a firebox comprising: aplurality of coils comprising inlet sections and outlet sections, withoutlet sections of the coils positioned in at least one lane and withinlet sections of the coils positioned in at least two lanes; and atleast two sets of burners, wherein one set of burners is located in acenter portion of an end wall of said fired heater and one set ofburners is located on a floor next to a side wall of said fired heater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of one-half of a fired heater according to theinvention.

FIG. 2 shows a prior art fired heater with side and center burners.

FIG. 3 shows a fired heater of the invention with side burners and floorburners.

FIG. 4 shows a schematic of heat distribution for a prior art firedheater.

FIG. 5 shows a schematic of heat distribution for a fired heater of thepresent invention.

FIG. 6 shows the deviation of heat within a prior art fired heater and afired heater of the present invention.

DESCRIPTION OF THE INVENTION

The present invention decreases heater coil and manifold hot volumethereby improving selectivity and minimizing pass to pass bulk outlettemperature variation from heater average outlet conditions whiledecreasing cost and complexity of equipment and decreasing heat losses.

Compared to a prior art configuration there is an 18% reduction in coilheat transfer surface area/volume, 32% reduction in firebox volume, 42%reduction in casing area of firebox and flue gas ducts, outlettemperature variation reduced by a factor of 5 and equipment costreduced by 10% plus. The invention provides significant reductions incapital and operating costs.

Double fired U-tube heaters use burners located on the end wall (wallperpendicular to radiant manifolds) inside and outside the U firinghorizontally towards the middle of the box. The opposing flames cause ahot spot in the middle of the box causing high outlet temperatures inthat region while passes towards the end walls have lower outlettemperature. The tubes in the middle of the box set the surface arearequirements for these film/tube wall temperature limited heaters. Priorart has utilized insulating tubes in the middle to somewhat decreaseeffect of hot spot but the hot volume/surface area requirements areunaffected by this approach.

This invention relocates the side burners (burners outside U-tube) tofloor firing positions vertically upwards and biasing their placementtowards the end walls. This eliminates their contributing to the hotspot in the middle of the box and their new placement adds heat topasses away from the middle (these passes pick up less heat intraditional design) thereby minimizing heat pickup variation amongstpasses and permitting lower coil surface area and other savingsidentified above. A particular new feature is the combination of center(inside U) end wall horizontal fired burners with floor mounted,vertically upfired side burners biased towards the end wall. The floormounted, vertically upfired side burners may be biased

FIG. 1 is a schematic of one-half of a two cell fired heater. Thedrawings is not to scale. Cell A is shown, while Cell B is not shown.This heater 10 is symmetrical about the centerline 32 as shown. Heater10 has an outer wall 12. Inlet manifold 14 and outlet manifold 16 areshown for the flow of air being heated as it passes through U-shapedcoil 18. While only one U-shaped coil is shown in FIG. 1, inside a firedheater will be found many parallel U-shaped coils throughout the heater.Inside U-shaped coil 18 are shown four center burners 22, 24, 26 and 28that are mounted on the end wall and are generally horizontally fired.Side burners 34, 36 and 38 are shown mounted on the floor and aregenerally vertically up fired, although they may be either oriented tofire in the direction of the end wall or into the interior of the firedheater. Flue gas 34 is seen exiting at 30. Access doors, platforms,manifold supports, instrument connections etc. are not shown in FIG. 1for clarity. One column of center burners at each end wall with fourlevels of burners per column is shown. However, it would be possible touse two columns of burners at each end wall with a number of levels ofburners as needed. A combination of two columns of burners at certainlevels and one column at other levels is another configuration thatwould be feasible. As depicted, the side burners fire against side wall(i.e. burners are immediately next to side wall and the flames are laidagainst the wall). However, side burners could also be free-standingwhere the burners are further away from the side wall and the flame isnot against the wall. The side burner placements may be biased towardseach end wall to even out heat flux mal-distribution. The number of sideburners and extent of bias is optimized as needed.

As depicted, inlet manifold 14 of U-tube coil 18 is located away fromheater centerline 40 and outlet manifold 16 is closer to the heatercenterline. However, the inlet and outlet manifold locations may beswapped as needed independently in each cell. As depicted, an airwallseparates the two cells in the heater. Alternately, a firewall (such asa brick wall) may be used.

Two cells per heater are implied in the drawing. However, theconfiguration of the present invention can be used for one cell or for amultiple cell heater that has two or more cells.

FIG. 2 is a prior art fired heater that has center burners and sideburners. Process gas inflow and outflow is shown as well as flue gasexiting from the top. In this fired heater, center burners 100 are shownon a front side with flames from the heaters directed to the interior ofthe fired heater. Side burners 102 are shown on either side of the twosets of center burners 100. Process gas is shown flowing into burner at104 and 110 and flowing out of the burner at 106 and 108. Flue gas 112exits from a top opening of the fired heater. There are a series ofcoils within the fired heater that are not shown in this view.

FIG. 3 shows a fired heater of the present invention that is morecompact and differs from the prior art fired heater by having flat-flamefloor burners shown on the floor by the side walls of the fired heater.There are center burners that are in the same position as center burners100 from FIG. 2. Instead of the side burners, there are now shownmultiple sets of flat-flame floor burners 114 with pairs of burners onthe bottom of each long side with a total of eight pairs of flat-flamefloor burners. There are shown process gas inflow 116 and process gasoutflow 118 and 122. There is a central ducting shown that is reduced incomparison to FIG. 2. Flue gas exits at 124.

FIG. 4 shows a prior art fired heater, with a central section has anincreased temperature while in FIG. 5 there is a significantly improveduniformity of heat within the fired heater. A series of coils 200 areshown in the center portion of the firebox for FIG. 4 with burners atboth ends of the enclosure. In FIG. 5, the burners 206 are on the sidesas well as a single burner 204 shown at each end and adjacent to coils200.

FIG. 6 shows the deviation from average heat as measured at coils 1-32from the end wall. The prior art fired heater configuration shows muchgreater variation with as much as a total of 26 F difference than thepresent invention which shows minimal variation with only about 4 Fdifference.

There are a variety of fired heaters that are designed for differentuses including general refinery service. ANSI/API Standard 560, Fourthedition, August 2007 includes different configurations for firedheaters, especially for the lay-out of the coils and locations for theburners. Coils may be in a U-shape, a helical configuration, a series ofhorizontal tubes, or there may be vertical tubes. Burner arrangementsare shown in this standard as up-fired, endwall-fired, sidewall-firedand sidewall-fired multilevel.

Suitable cracking coils (also referred to as cracking tubes), aregenerally known. The coils may be formed of one or more cylindricaltubular conduits, preferably with a circular or oval cross-section. Theconduits may be connected by connecting devices such as but not limitedto connecting tubes and bends to provide a number of passes. A crackingcoil may be formed of a plurality of tubular conduits joined together,for example having an “m-like shape” or “w-like shape” wherein the outerlegs represent inlet sections which mount in a single outlet section,represented by the central leg of the w/m.

The coils generally each have at least one inlet and at least one outletThe inlet of the coil is a conduit via which, during use, the feedenters the cracking coil and usually thereby the firebox; the outlet isthe conduit via which, during use, the product leaves the cracking coil,and thereby usually the firebox. The outlet may be connected with otherprocessing equipment such as but not limited to heat exchangers and/orquenchers.

The inlet section of a coil is the first part (in the longitudinaldirection) of the coil that is inside the firebox, starting from theinlet of the coil into the firebox. It may extend up to the beginning ofthe outlet section. In particular, it is the part that is less thermallyshielded than the outlet section. In a preferred embodiment, the inletsection is the part of the coil that thermally shields the outletsection of the coil, when operating the furnace.

The outlet section of a coil is the last part (in the longitudinaldirection) of the coil that is inside the firebox, ending at the outletof the coil going out of the firebox. In particular it is the part thatis more thermally shielded than the inlet section. It may extend up tothe end of the inlet section or to an intermediate section connectinginlet section and outlet section (such as return bends, as will bediscussed below).

Usually, a plurality of the cracking tubes are connected to each otherto form a parallel flow path for the feed. Thus, in contrast to a designwherein the “tubes” are connected in a serial manner and wherein thefeed enters a first “tube”, is partially converted and thereafter entersa subsequent “tube”, the present design allows the composition of thestream at the inlet of each tube to be essentially the same for eachtube. This allows short residence time and thereby high through put. Ifdesired, during use, a plurality of the cracking tubes may thus be fedfrom a single container or conduit that is split into a number of feedstreams, each fed to the inlet of a cracking tube and/or the productstream leaving the plurality of tubes via the outlet may be combinedagain into a single conduit or container.

The term that an entity (such as a coil section) is “thermally shielded”is defined herein as heat, being hindered to be transferred into theentity. This term is in particular used herein to indicate the extent towhich heat generated by the burners during operation of the crackingfurnace is hindered to be transferred into the shielded entity. Withrespect to the outlet sections of the coils being more thermallyshielded than the inlet sections of the coils, this means in particularthat the heat transfer into the cracking coils at the outlet section ofthe coil is shifted in favor of the heat transfer into the crackingcoils at the inlet section of the coil, during operation of the burnerscompared to a coil configuration whereby such shielding is not or lessoccurring.

The term essentially vertically is used herein to indicate that anentity (such as a coil/tube or part thereof, a lane, a wall, etc) atleast during use is at an angle of more than 45° with a horizontalsurface (usually the floor of the firebox), in particular at an angle ofmore than 80°, preferably at an angle of about 90°.

The term essentially horizontal is used herein to indicate that anentity (such as a coil/tube or part thereof, a lane, a wall, etc.) atleast during use, is at an angle of less than 45° with a horizontalsurface (usually the floor of the firebox), in particular at an angle ofless than 10°, preferably at an angle of about 0°.

The term essentially parallel (used in the geometrical sense) is usedherein to indicate that an entity (such as a tube or part thereof, alane, a wall, etc.) at least during use, is at an angle of less than 45°with another entity to which the entity is said to be essentiallyparallel, in particular at an angle of less than 10°, preferably at anangle of about 0°.

The term “about” and the like, as used herein, is in particular definedas including a deviation of up to 10%, more in particular up to 5%.

A process according to the invention respectively a furnace of theinvention may offer several advantages.

The hydrocarbon feed to be heated may be any gaseous, vaporous, liquidhydrocarbon feed or a combination thereof. Examples of suitable feedsinclude ethane, propane, butanes, naphthas, kerosenes, atmospheric gasoils, vacuum gas oils, heavy distillates, hydrogenated gas oils, gascondensates and mixtures of any of these.

Specific Embodiments

While the following is described in conjunction with specificembodiments, it will be understood that this description is intended toillustrate and not limit the scope of the preceding description and theappended claims.

A first embodiment of the invention is a fired heater comprising afirebox comprising a plurality of coils comprising inlet sections andoutlet sections, with outlet sections of the coils positioned in atleast one lane and with inlet sections of the coils positioned in atleast two lanes; and at least two sets of burners, wherein one set ofburners is located in a center portion of an end wall of the firedheater and one set of burners is located on a floor next to a side wallof the fired heater. An embodiment of the invention is one, any or allof prior embodiments in this paragraph up through the first embodimentin this paragraph, the lanes are about parallel to each other. In yetanother embodiment, the outlet sections and the inlet sections arepositioned about vertically, at least during use. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the first embodiment in this paragraph, the inlet sections ineach lane of the at least two lanes and the outlet sections in the atleast one lane are arranged in an inline arrangement or in a staggeredarrangement, and the inlet sections in each lane of the at least twolanes are positioned in a staggered configuration with respect to outletsections present in an adjacent parallel lane. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the first embodiment in this paragraph, the burners on the floorhave a flame that is directed in a vertical direction. An embodiment ofthe invention is one, any or all of prior embodiments in this paragraphup through the first embodiment in this paragraph, the burners on thefloor have a flame that is directed in a horizontal direction. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph,wherein there are at least one set of burners above the burners on thefloor. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph, wherein the burners next to the side walls are immediatelyadjacent to the side walls.

A second embodiment of the invention is a method of improving heatdistribution in a fired heater comprising providing a fired heatercomprising a firebox comprising a plurality of coils comprising inletsections and outlet sections, with outlet sections of the coilspositioned in at least one lane and with inlet sections of the coilspositioned in at least two lanes; and at least two sets of burners,wherein one set of burners is located in a center portion of an end wallof the fired heater and one set of burners is located on a floor next toa side wall of the fired heater.

1. A fired heater comprising a firebox comprising: a plurality of coils comprising inlet sections and outlet sections, with outlet sections of the coils positioned in at least one lane and with inlet sections of the coils positioned in at least two lanes; and at least two sets of burners, wherein one set of burners is located in a center portion of an end wall of said fired heater and one set of burners is located on a floor next to a side wall of said fired heater.
 2. The fired heater of claim 1 wherein the lanes are about parallel to each other.
 3. The fired heater of claim 1 wherein the outlet sections and the inlet sections are positioned about vertically, at least during use.
 4. The fired heater of claim 1 wherein the inlet sections in each lane of the at least two lanes and the outlet sections in the at least one lane are arranged in an inline arrangement or in a staggered arrangement, and the inlet sections in each lane of the at least two lanes are positioned in a staggered configuration with respect to outlet sections present in an adjacent parallel lane.
 5. The fired heater of claim 1 wherein the burners on the floor have a flame that is directed in a vertical direction.
 6. The fired heater of claim 1 wherein the burners on the floor have a flame that is directed in a horizontal direction.
 7. The fired heater of claim 1 wherein there are at least one set of burners above said burners on the floor.
 8. The fired heater of claim 1 wherein said burners next to said side walls are immediately adjacent to said side walls.
 9. The fired heater of claim 1 wherein said burners next to said side walls are extended away from said side walls.
 10. A method of improving heat distribution in a fired heater comprising providing a fired heater comprising a firebox comprising: a plurality of coils comprising inlet sections and outlet sections, with outlet sections of the coils positioned in at least one lane and with inlet sections of the coils positioned in at least two lanes; and at least two sets of burners, wherein one set of burners is located in a center portion of an end wall of said fired heater and one set of burners is located on a floor next to a side wall of said fired heater. 